1. Field of the Invention
The present invention relates to a method of and a device for compensating for a reading-position error of an image sensor, and it also relates to an improved image scan reader and an improved device for inspecting a pattern of a printed circuit board.
2. Description of the Background Art
Printed circuit boards are employed in the field of electronic engineering for mounting and interconnecting electronic elements, and are provided with conductive wiring patterns on one or both sides of insulating boards and with a large number of through holes piercing the insulating boards. Various types of optical visual inspection devices or pattern inspection devices have been employed in order to inspect whether or not the conductive pattern and the through holes are formed accurately within a tolerance.
FIG. 18 is a schematic view of an optical head 951 used for a pattern inspection device of this type. The optical head 951 includes a linear image sensor 952 and an image-formation lens system 953. A printed circuit board 950 and the linear image sensor 952 are moved relatively, and thereby the image of the printed circuit board 950 can be read for each scanning line.
FIG. 19 shows a pattern inspection device of a multi-channel type in plan view. As shown in FIG. 19, a printed circuit board 950 is placed on a movable table 954. The device is provided with a plurality of optical heads 951a to 951c having the same structure as the optical head 951 of FIG. 18. The optical heads 951a to 951c read an image while the movable table 954 is transferred in the (-.alpha.) direction. Accordingly, the image of the printed circuit board 950 is read serially in the direction .alpha..
Positions 952a to (52c which image sensors in the optical heads 951a to 951c read at the same time (hereinafter referred to as "reading-positions") are not on a straight line, because of the mounting errors of the image sensors in the optical heads 951a to 951c and/or because of errors in mounting the optical heads 951a to 951c to an optical head support frame. Such errors because the reading-positions 952a to 952c cause a time lag between image signals outputted from the optical heads 951a to 951c, so that accurate image information cannot be obtained.
Hence, there arises a necessity for compensating for the reading-position errors.
Conventionally, the reading-position errors have been compensated in a manner described below. The hindmost reading-position 952b as viewed in the .alpha. direction is taken as a reference position PR, to measure positional deviations e.sub.a and e.sub.c of the other reading-positions 952a and (52c from the reference position PR. Signals outputted from the optical heads (51a and 951c are delayed for the times corresponding to the position deviations e.sub.a and e.sub.c, respectively. Through this processing, obtained is a condition equivalent to the imaginary state that all of the linear image sensors read the image of the printed board 950 at the reference position PR.
In such a pattern inspection device for printed circuit boards, the image which is obtained by the optical heads 951a to 951c or the image obtained by processing the read image is stored in a storage medium. The image of a printed circuit board having no pattern defects is delivered to other devices of the same type through the storage medium, and is employed therein as a reference image for a pattern comparative inspection and the like.
It is therefore preferable that the image obtained by one device is employed intactly in other devices. That is, the interchangeability of image information is necessary among a plurality of devices.
In the conventional method, however, each device sets the reference position PR individually. This is because the hindmost reading-position as viewed in the scanning direction .alpha. depends on the mounting error inherent in each of the devices. The conventional method has not accomplished the aforesaid interchangeability sufficiently. When the image information is transmitted from one device to another device, the conventional method needs position correction in accordance with the relative shift of the reference position for each device.
A similar problem arises where each of the optical heads includes a plurality of image sensors. Even when the reading-position errors are compensated in each optical head, the errors are not compensated in relation to other optical heads. The matching of the image informations provided by the respective optical heads cannot be ensured.
The problem of the interchangeability of the image informations occurs also between the pattern inspection devices each having a single image sensor. The image information obtained in one device cannot be used intactly in other devices because of variation in the mounting position of the image sensor.
Not only the pattern inspection device for the printed circuit board but also various image scan readers have the above-mentioned problems in common.